I see your point, but I think that Jedi are there as exmplars of justice, if they start becoming judge, jury and executionor (particular when, at least to the Jedi in question the fight is over and the Sith is begging for his/her life) to the kill them means that Jedi has lost their way somewhat. After all, if the Jedi go around deciding when they should uphold justice or should just take it into their own hands as they see fit, I would say they are already on the path to being in some ways little different to the Sith, arbitarily deciding the fates of all beings lives based on their personal desires.